Receiving circuit for telegraph signaling systems



J. R. DAVEY March 7, 1944.

RECEIVING CIRCUIT FOR TELEGRAPH SIGNALING- SYSTEMS Filed Sept. 25, 1942u :55 0mm i I'll INVENTOR J. R. DA V5) ATTORNEY Patented Mar. 7, 1944RECEIVING CIRCUIT FOR TELEGRAPH SIGNALING SYSTEMS James R. Davey, NewYork, N. Y.. assignor to Bell Telephone Laboratories, Incorporated. NewYork, N. Y., a corporation of New York Application September 25, 1942,Serial No. 459,672

2 Claims.

The invention relates to telegraph signal transmission systems andparticularly to the signal receiving circuits used in such systems.

The invention is particularly applicable to a carrier telegraph systemin which the spurts or pulses of alternating carrier current transmittedover the system represent marking signals and the intermediate intervalsof no carrier represent the spacing signals, and in which the receivingcircuit of the system includes one or more stages of vacuum tubeamplification for the received carrier pulses followed by a signaldemodulator and a polarized relay controlled from the output of thelatter for repeating the detected marking and spacing signals. It hasbeen found that the signals transmitted over such a system undergo aform of distortion, known as "telegraph bias, which is evidenced by adiflerence in the durations of the marking and spacing signals asrepeated by the receiving relay from those which they had at thetransmitting end of the system, due mainly to changes in the level ofthe received carrier caused byvariations in line equivalent, and to alesser extent due to changes in battery voltages and receiving relayadjustments. Telegraph bias has been minimized by the use of automaticcontrol circuits, known as level compensators, operating to apply avariable bias to the control grid of one or more of the vacuum tubes inthe circuit to adjust the gain of the receiving circuit in accordancewith the amplitude level of the detected signals.

An object of the invention is to provide improved operation of suchtelegraph receiving circuits.

A more specific object is to reduce signal dis- I tortion in a telegraphtransmission system including that type of distortion known as telegraphbias.

These objects are attained in accordance with the invention partly byderiving the variable bias applied to the control grid of one or moreamplifier tubes in the receiving circuit to adjust their gain for levelcompensation, from the control grid circuit of an amplifying tetrodeconnected between the demodulator and the polar receiving relay; and inpart by a special arrangement of the biasing and operating windings ofthe polar receiving relay so as to provide a more rapid transition frommark to space and vice versa.

The various objects and features of the invention will be betterunderstood from the following detailed description when read inconjunction with the accompanying drawing in which:

Fig. 1 shows schematically a receiving circuit for a voice frequencycarrier telegraph system embodying the invention; and

Fig. 2 shows curves illustrating the operation of the level compensatorin the circuit of Fig. 1.

The receiving circuit of the invention is shown in the drawingincorporated in one channel, A, of a voice frequency carrier telegraphsystem. The input of the receiving channel A (illustrated in detail) ofthe carrier telegraph system of Fig. l is connected in parallel with theinputs of the other receiving channels B, C to the tele graph line Lthrough the 1:1 ratio transformer or line coil I used primarily toseparate the line, which is balanced to ground, from the receiving bandfilters which are unsymmetrical networks with one side grounded. Thereceiving channel A includes in its input a band-pass filter 2, one sideof which is grounded as shown, for selecting from the carrier modulatedtelegraph signal waves received over the transmission line L, theparticular frequencies assigned to that channel. The output of thefilter 2 is connected by an input transformer 3, which gives voltageamplification and serves to isolate the rest of the circuit from theground on the receiving filter 2, to the input of an amplifiercomprising two pentode amplifying vacuum tube stages 4 and 5 coupled intandem by the resistance-condenser coupling circuit 8. The first stagetube 4 is operated with a variable control grid bias, and the secondstage amplifier tube 5 is operated with a fixed control grid bias sothat it serves as a simple amplifier. The potentiometer l in the inputof the amplifier tube 4 is provided to regulate manually the strength ofthe incoming signals.

The plate-cathode circuit of the amplifier tube 5 is coupled by outputtransformer 8 across the input diagonal of the full-wave copper-oxiderectifier bridge 9 and the resistance Ill and parallel condenser Hprovided for filtering out the alterhating current component of thedetected signals are shunted across the output diagonal of the latter.The control grid-cathode circuit of the amplifying tetrode I2 is coupledthrough the series resistance 13 and shunt resistance ll across theoutput of the rectifier 9, and is coupled by a feedback circuitincluding the condenser-resistance network II to the controlgrid-cathode circuit of the first tube 4 of the alternating currentamplifier to provide the variable bias on that tube for levelcompensation.

The operating winding l5 and the opposing biasing winding it of thepolar receiving relay RR are connected in series between the plate andcathode of the amplifying tetrode l2, and both windings are connected tothe plate battery II of that tube through a rather large commonresistance 2B. The screen grid of the tetrode I2 is positively biased bythe plate battery ll through the rheostat ll shunted around the biasingwinding ll of relay RR. and reistance 2B in series.

The other elements of the circuit of the invention as shown in thedrawing will be referred to in the following complete description ofoperation.

When no carrier current has been received from the line L for some time,the control grid oi the first tube 4 oi the alternating currentamplifier is unbiased and the cathode oi the direct current amplifiertube I2 is positive with respect to the control grid of that tubebecause 01' the voltage drop produced in the resistance II by currentfrom the plate battery I'I. Under these conditions, the plate current ofthe direct current amplifier tube l2 and hence the current transmittedthrough the operating winding ll of the polar relay RR are negligible.On the other hand, current of about 17 mils fiows through the biasingwinding it of the relay RR from battery i'i, causing the armature ofthat relay to be held firmly on the spacing contact 8.

The incoming carrier wave modulated in ac cordance with the marking andspacing signals, generated at the transmitting end or the system (notshown) received over the transmission line L is impressed by the linecoil l on the input of the receiving channels A, B, C. That portionwithin the frequency range assigned to the receiving channel A will beselected by the receiving band-pass filter 2 therein, and the selectedwaves will be impressed by the transformer I on the control grid-cathodecircuit of the first pentode amplifying stage 4 of the alternatingcurrent amplifier. The impressed carrier pulses will be amplified by theamplifying tubes 4 and I of the alternating current amplifier, and theam-- plified waves will be applied by the output transformer 8 to theinput of the copper-oxide rectifier bridge 9 and will be rectifiedtherein. The resultant direct current voltage across the resistance Idue to rectification of the carrier pulses in copper-oxide rectifierbridge 9, will be impressed between the cathode and the control grid ofthe direct current amplifier tube i2. This positive voltage will belarge compared to the negative bias on the control grid of that tube,and hence the control grid-cathode space of tube i2 becomes conductive.The resultant control grid current through the resistance will cause thecondensers 2|,. 22 and 23 in the condenserresistance network H to becharged. The voltage drop across the condenser 2| is in a direction toadd to the original bias between the oathode and the control grid of thedirect current amplifier tube i2 causing that grid to become somewhatmore negative, but the efiect on the operation of the tube is veryslight because the change in control grid bias is but a small fractionor the whole. Part of the voltage across the condenser 2| appears acrossthe condenser 22 and is impressed on the control grid of the alternatingcurrent amplifier tube 4.

The signaling voltage impressed on the control grid-cathode circuit orthe amplifier tube 4 across the potentiometer I is quite small comparedto the total range of the tube characteristic. The relation is showngraphically in Fig. 2. By changing the bias 1:, the point of operationon the characteristic may be shifted. It

willbeevidentthatthegreaterbisinthenega tive direction, the smaller thecurrent i will for a given value of voltage 0. Hence, it

input voltage increases due to a decrease inlossoilineLtoavaluesuchasvs,theresuiting current is may be keptidentical with it by changing the bias irom in to in. The amplifier tube4,

may be made to act as a transmissionlevel compensator by providing itwith a suitable varying control grid bias. The control grid oi the tube4 is negative with respect to its cathode under all operatingconditions.

Itwill be clear from the foregoing that, in so far as level compensationis concerned. the tetrode l2 serves principally as a means for producing a variable biasing voltage, while the pentode tube 4 does theactual regulation by acting as an amplifier of varying amplificaon Aftera very long spacing signal, the condensers 2|, 22 and 22 in thecondenser-resistance network i4 will all be discharged and thesensitivity of the amplifier tube 4 will be at a maximum. If the distantstation associated with the other end of the telegraph line L sends amarking signal under these conditions, a large voltage will be appliedto the control grid-cathode circuit of the amplifier tube l2, a heavycontro1 grid-cathode current resulting and the condenser 2| chargesquickly to a voltage which is excessive, because the control tube 4 hasnot had time to act. The proper biasing voltage to apply to the controlgrid of the amplifier tube 4 is, therefore, less than that across thecondenser 2|. This is taken care of by the action of the voltage dividerformed by the two condensers 22 and 2: in which the condenser 22 hasonly about one-twelfth of the capacity of the condenser 24. Hence, atfirst only onetwelith of the voltage across the condenser 2| is appliedto the control grid of the tube 4. This reduce its gain somewhat. Ascurrent continues coming in, the condenser 22 gradually dischargesthrough the parallel resistance 24, thereby increasing the voltageacross the condenser 23 and iurther reducing the amplification or theamplifier tube 4. Condenser 2| discharges through the parallelresistance 20 at about the same rate. since the two circuits haveapproximately equal time constants. Ir marking is maintained for asufiicient length of time, the condenser 22 will become i'ullydischarged while the voltage across the condenser 22 will become equalto that across the condenser 2| without objectionable oscillationsdeveloping in the control grid bias of the tube 4 due to "huntingbetween the charges on condensers 2| and 22. During the transmission ofordinary telegraph s 'nal text, there is a tendency for the levelcompensator condensers 2| and 24 to discharge during the spacingintervals and to charge during the marking intervals, but the voltageacross the condenser 2|, which is a large condenser, does not changemuch, while that across the condenser 22 changes a great deal less. Theeffect of the condenser 22 is to screen the condenser 23 from suddenchanges in the voltage of the condenser 2| while permitting thecondenser 22 to assume the same average voltage as the condenser 2|.This is brought about because the resistance 24 in parallel with thecondenser 22 is unresponsive to rapid voltage changes and for such rapidvoltage changes acts as if it were not there so that the voltage dividesin inverse proportion to the capacity oi the condensers 22 and 22, wherethe condenser 23 is large compared to condenser 22. However, in the caseor slow changes in voltage across the condenser 2|,

such as will result from variations in the transmission loss of the lineL, the resistance 24 will act practically as a short circuit around thecom denser 22 and the voltage of the condenser 23 will follow these slowvoltage changes faithfully. An advantage of the condenser-resistancearrangement I4 is that hits, such as result from distant lightning, havelittle eflect on the control grid bias of the amplifier tube 4.

After rectification and. amplification, the envelopes of the carriersignals supplied from the input of the receiving filter 2 in thereceiving channel A appear as direct current pulses closely similar tothose originating in the sending channel at the other station. Thesedirect current pulses will pass from the plate of the amplifier tube I!through the operating winding l5 of the polar relay RR which is biasedto its spacing contact S by the normal current flowing through thebiasing winding l6 from battery l1, causing the operation or the relayto its marking contact M. Because of the large resistance 25 connectedbetween the mid-point of the operating winding l5 and the biasingwinding [6, and the positive terminal of the plate battery H, as theplate current of the amplifier tube l2 increases the biasing current inthe biasing winding l6 of the polar relay RR will decrease. The variousresistances shown in the circuit of the biasing winding [6 areproportioned so that when no carrier current is received and the platecurrent of the amplifier tube I2 is zero, the baising current throughthe biasing winding it of relay RR has a maximum value of about 17 mils,while when the plate current of tube l2 rises to its maximum value whichwas about 25 mils in the receiving circuit which has been constructed,the biasing current is very small. The two currents equalize when eachis about 10 mils. This, therefore, corresponds to the operating pointoi! the relay. The arrangement of the biasing and operating windings ofthe relay RR which has been described, provides a rapid transition inthe magnetizing force operating on the relay armature as the signals gofrom "mar to "space" and vice versa, which favors distortionlessoperation. The

point of operation of the relay RR may be varied over a considerablerange to secure unbiased signals by changing the screen grid voltage ofthe amplifier tube I2 by adjustment or the movable arm of rheostat l9.It will be noted that with this arrangement zero bias is obtained byvarying the operating current of the relay to compensate for a fixedbias instead of the reverse as is usually the case.

The milliammeter M shown as connected across the series resistance 26 inthe plate circuit of the amplifier tube l2 by the switch may be used forreading directly the current passing through the operating winding I 5of the polar relay RR.

As pointed out above, the amplifier I2 is arranged to give asubstantially constant current into the receiving relay RR. for aconsiderable range of variation in its input. Since increases anddecreases in its input level due to variations in the loss of the line Lare equally likely to happen, the sensitivity or the detector 0 shouldbe adjusted to correspond to approximately the middle of thecompensation range when the level of the received current is at itsnormal value.

Various modifications of the circuits illustrated and described whichare within the spirit and scope of the invention will occur to personsskilled in the art.

What is claimed is:

1. In a telegraph system in which pulses of carrier current separated byintervals of no current are used for marking and spacing telegraphsignals, respectively, a transmission medium subject to variable lossfor transmitting said signals and a signal eceiving circuit connected tosaid medium including a rectifier for rectifying the received signalpulses, an electron discharge am plifying device having electrodesincluding a cathode, an anode and a control grid, and circuits therefor,a source of anode current for said device, means for applying therectified signal pulses to the control grid-cathode circuit of saidamplifying device, a large resistance, a polar relay for repeating themarking and spacing signals amplified by said amplifying devicecomprising a relay armature, an operating winding and an oppositelypoled biasing winding connected in series between the anode and cathodeof said device, and to said anode battery in common through said largeresistance so that when the current through the operating winding of therelay increases during marking signaling intervals the biasing currentto said biasing winding decreases, to provide a more rapid transition inthe magnetizing force operating on the relay armature as the signals gofrom mark to space and vice versa.

2. A receiving circuit for a carrier telegraph system transmittingpulses of alternating carrier current separated by intervals of nocurrent respectively representing marking and spacing telegraph signals,comprising an amplifier for amplifying the received carrier pulses, arectifier for rectifying the amplified pulses, an electron dischargeamplifying device having a control gridcathode circuit supplied with therectified pulses, and an anode-cathode circuit including a source ofspace current, a polar telegraph relay having an armature, marking andspacing contacts, a biasing winding normally energized from said sourceto maintain the relay armature on its spacing contact, and an operatingwinding energized by the output current of said amplifying electrondischarge device in response to the rectified marking signals applied tothe control gridcathode circuit of said device, to operate said armatureto its marking contact, the operating and biasing windings of said relaybeing. connected to said space current source through a relatively largecommon resistance so that the biasing current flowing in said biasingwinding decreases as the operating current in said operating windingincreases, thus providing a more rapid transition in 'the magnetizingforce applied to said relay armature when operated from said spacingcontact to said marking contact and vice versa.

JAIMES R, DAVEY.

